One way to make quick and easy rocks and boulders is to insert a sphere into your scene, then apply one of the Rocks and Stone category shaders from the browser. After you've done that,  open up the shader in the shader room and click on the displacement tab. Once there make sure Enable Displacement is checked, then click on the Wizard button and select Noise. Viola! Instant boulder. Now you can scale the different axis, make 'em smaller or larger, rotate and tilt 'em. They can even be added to a surface replicator for mass quantities.